1. Field of the Invention
The present invention relates to an improvement of a bearing unit for supporting a wheel of an automobile as well as a rotary body for braking such as a rotor or a drum, and to an improvement of a method of manufacturing such bearing unit for the wheel.
2. Related Background Art
A wheel 1 constituting a traveling wheel of an automobile and a rotor 2 constituting a disk brake classified as a braking device, are rotatably supported by a knuckle 3 as a component of a suspension on the basis of, for example, a structure shown in FIG. 4. To be more specific, a circular support hole 4 is formed in the knuckle 3. An outer ring 6 defined as a static ring constituting a bearing unit 5 for a wheel, to which the present invention is applied, is fixed to within the support hole 4a by use of a plurality of bolts 7. On the other hand, the wheel 1 and the rotor 2 are fixedly joined through a plurality of studs 9 and a plurality of nuts 10 to a hub 8 as a component of the wheel bearing unit 5.
Outer ring double raceways 11a, 11b each serving as a static-side raceway surface are formed in an inner peripheral surface of the outer ring 6, and a joint flange 12 is provided on an outer peripheral surface thereof. The thus configured outer ring 6 is fixed to the knuckle 3 by joining the joint flange 12 to the knuckle 3 with the bolts 7.
By contrast, a fitting flange 13 is provided at a portion protruding from an aperture of an outer side end (the term “outer or external” implies an outside portion in a widthwise direction in a state of being assembled to the automobile, i.e., the left side in each drawing. In contrast, the term “inner or internal” implies a central portion in the widthwise direction in the state of being assembled to the automobile, viz., the right side in each of the components.) of the outer ring 6. The wheel 1 and the rotor 2 are fixedly joined through the studs 9 and the nuts 10 to one single surface (an external surface in the illustrated example) of the fitting flange. Further, an inner ring raceway 14a is formed in a portion, facing to the more external raceway 11a of the outer ring double raceways 11a, 11b, of the outer peripheral surface of the intermediate portion of the hub 8. Moreover, the inner ring 16 is fixedly fitted onto a small-diameter stepped portion 15 provided at an inner side end of the hub 8. Then, the inner ring raceway 14b formed in the outer peripheral surface of the inner ring 16 is disposed facing to the more internal raceway 11b of the outer ring double raceways 11a, 11b. 
Balls 17, 17 each defined as a rolling member are provided in a rollable manner by pluralities between the outer ring raceways 11a, 11b and the inner ring raceways 14a, 14b in a state of being held by retainers 18, 18. Based on this construction, a double-row angular ball bearing with a back face combination is structured, the hub 8 is rotatably supported in the outer ring 6, and a radial load and a thrust load are supported in a sustainable manner. Note that seal rings 19a, 19b are provided between inner peripheral surfaces of two side ends of the outer ring 6, an outer peripheral surface of an intermediate portion of the hub 8 and an outer peripheral surface of an inner side end of the inner ring 16, thus cutting off a space accommodating the balls 17, 17 from the outside space. Further, in the illustrated example, the wheel bearing unit 5 is structured for a drive wheel (corresponding to a rear wheel of a FR car and an RR car, a front wheel of an FF car, and whole wheels of a 4WD car), and hence a central portion of the hub 8 is formed with a spline hole 20. Then, a spline shaft 22 of a constant velocity joint 21 is inserted into the spline hole 20.
When using the roll bearing unit 5 for the wheel described above, as shown in FIG. 4, the outer ring 6 is fixed to the knuckle 3, and the rotor 2 and the wheel 1 assembled with an unillustrated tire are fixed to the fitting flange 13 of the hub 8. Further, the rotor 2 of those components is combined with an unillustrated support and an unillustrated caliper which are fixed to the knuckle 3, thus constituting a disk brake for braking. When braking, a pair of pads provided with the rotor being interposed therebetween are pressed against the two side surfaces of the rotor 2.
It is a known fact that vibrations called judder followed by disagreeable or annoying noises often occur when braking the wheels of the automobile. One of a variety of known causes of those vibrations may be an ununiform state of frictions between the side surface of the rotor 2 and the lining of the pad, however, deflections of the rotor 2 are also known as another large cause. More specifically, the side surface of the rotor 2 should be fundamentally right-angled to a rotational center of the rotor 2, however, a perfect perpendicularity is hard to obtain due to an inevitable manufacturing error. As a result, the side surface of the rotor 2 inevitably deflects more or less in directions of the rotational axis (right and left directions in FIG. 4) when the automobile travels. If such deflections (displacement quantities in the right and left directions in FIG. 4) increase, and if the linings of the pair of pads are pressed against the two side surfaces of the rotor 2 for braking, the judder occurs. Then, the contact between both surfaces of the rotor 2 and the respective linings of the pad becomes non-uniformed, wherein a partial abrasion of the linings is caused.
It is of much importance for restraining the judder occurred by such causes to decrease (axial) deflections of the side surface of the rotor 2 (to enhance the perpendicularity of the side surface to the central axis of rotation). Then, it is required that a perpendicularity of a fitting surface (one side surface of the fitting flange 13) of the fitting flange 13 to the rotational center of the hub 8 and a surface accuracy of the fitting surface itself be enhanced for restraining those deflections. Factors for exerting influences upon the perpendicularity and the surface accuracy might exist by pluralities, however, the factors having an especially large influence may be a parallelism between the fitting surface and the raceway surface (between the outer ring raceways 11a, 11b and the inner ring raceways 14a, 14b) with respect to the perpendicularity, and a thermal process deformation with respect to the surface accuracy. Further, for enhancing the parallelism of those factors, among the constructive elements of the hub 8, there must be a necessity for setting highly accurately a positional relationship between the one side surface of the fitting flange 13, the inner ring raceway 14a formed in the outer peripheral surface of the intermediate portion and the small-diameter stepped portion 15 provided at the inner side end, and configurations and dimensions of those components. If the accuracy of the configurations and the dimensions of the inner ring raceway 14a and of the small-diameter stepped portion 15 among those elements are enhanced in the above relationship with the fitting surface, the perpendicularity of the fitting surface to the rotational center of the hub 8 can be enhanced. Moreover, if removing the thermal process deformation out of the fitting surface, the surface accuracy of this fitting surface can also be enhanced.
A technology for preventing the deflection of the fitting flange 13 which might conduce to the deflection of the rotor 2 may be what is disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 10-217001. The prior art disclosed in this Publication does not, however, deal with anything about thermally processing the respective components, wherein the costs wastefully increase because of precisely finishing the surface which is not essentially needed as the fiducial surface. While on the other hand, the inner ring raceway 14a and the small-diameter stepped portion 15 are hardened over their surfaces and therefore required to undergo the thermal process such as a high-frequency hardening process. Then, the configurations and dimensions of the inner ring raceway 14a and the small-diameter stepped portion 15 might more or less change subsequent to the thermal process, and hence, according to the prior art disclosed in the above Publication, it is difficult to sufficiently enhance the accuracy of each components in the way described above. Besides, the invention disclosed in the Publication given above takes a structure of fixing a couple of inner rings each separated from the hub to the outer peripheral surface of this hub, and therefore an error, etc. between the end surface of each inner ring and the inner ring raceway might be added as an error of parallelism between the fitting surface of the fitting flange and the inner ring raceway. Furthermore, a contact portion between the hub and the inner ring is not worked based on the fitting surface of the fitting flange, so that the parallelism between the fitting surface and the inner ring raceway is hard to sufficiently enhance.
Moreover, the bearing unit 5 for the wheel and the rotor 2 have hitherto been selectively combined in order to make offsets of the deflection of the fitting flange 13 and the deflection of the rotor 2 based on the configuration error of the rotor 2 itself. In this case, however, the selecting operation for the combination becomes laborious, resulting in an increase in cost.